The number of cellular phone users in the U.S. is now estimated at about 17 million and continues to grow at a rate exceeding 20 percent per year. Most of this growth in use has occurred in and around cities and towns with populations of 20,000 or more and has caused saturation of the channels available for cellular communication in many of these urban areas. Cellular service zones, referred to herein as "cellzones" for convenience, each servicing about 800 cellular telephone communications, have decreased in size from several miles in diameter to a few hundred meters in diameter as the density of cellphone users has increased apace in urban and suburban areas.
A cellphone subscriber will typically register use of a cellphone in a home cellzone and will temporarily re-register in one or more nearby cellzones if the subscriber makes or receives cellphone calls in these other cellzones. However, even where the cellphone subscriber is already registered in one or more nearby cellzones, the operator of any cellzone will typically be unaware of the presence of this subscriber in this cellzone, unless this subscriber has either recently requested cellular service in this zone or has somehow notified the operator that the subscriber is currently present in this zone. Knowledge of the number of cellphone subscribers currently in a given cellzone, who are not all active users of the cellular phone service at any given moment, may allow the service operator to rationally plan its present provision, and future expansion, of cellular service to accommodate the increasing demands.
A personnel locator system is disclosed in U.S. Pat. No. 3,439,320, issued to Ward. Each person carries a sensor/transmitter that emits a unique frequency whenever that person enters any one of a plurality of selected areas in a facility, with each such area having a receiver that receives this signal and transmits this information to a central processor. The processor keeps track of the movements of each person from one selected area to another area so that, at any given time, the processor can identify the selected area a given person now occupies. This approach is limited to a relatively small geographic area and to a relatively small number of covered persons.
Serrano et al disclose a microprocessor-controlled interface for a cellular phone system to be carried in a vehicle, in U.S. Pat. No. 4,718,080. A telephone handset communicates with the cellular transceiver along a bus and through the microprocessor that interprets bus logic signals, including voice and data signals. No integration is disclosed of the on-board cellular system with any instrument that reports vehicle operating information upon command received from a remote site.
U.S. Pat. No. 4,797,671, issued to Toal et al, discloses a motor vehicle locator system that indicates the location of a parked vehicle within a controlled area, such as a large parking lot. Each parked vehicle carries a receiver and audio/visual signal emitter. The receiver responds only to receipt of a signal having a unique frequency and causes the emitter to emit an audible or visually perceptible signal that indicates the present location of the parked vehicle. This approach requires that the vehicle be located within a controlled area of modest size, probably no larger than a city block.
Ando et al, in U.S. Pat. No. 4,837,700, disclose method and apparatus for processing and displaying the present location of a road vehicle, using a GPS receiver to provide data on vehicle location. This approach provides continuous information on vehicle location and vehicle orientation or vehicle movement vector based upon sensing the Earth's local geomagnetic field direction. An angular rate sensor determines, and optionally displays, the angular rate of change of vehicle orientation, and an odometer determines the distance the vehicle has moved over some predetermined time interval. The present vehicle location can be displayed graphically on an electronic map carried within the vehicle, with a sequence of maps being used to display the changing vehicle location. Map size can also be increased or decreased, and the displayed map can be automatically scrolled as vehicle position changes. A keyboard is available for entering instructions into a controller or processor. A reference location can be entered into the system, and vehicle location can be determined relative to this reference location. The location data can be stored if these data are acceptable and can be dumped into a bit bucket if these data are not acceptable.
In U.S. Pat. No. 4,924,402, Ando et al disclose a more detailed approach for determining and graphically displaying vehicle location using a GPS. This system uses a magnetic field sensor, an angular velocity sensor and a travel distance sensor together with GPS data and stores maps together with numerical data that describe useful features of the local terrain. Sensing of vehicle distance traveled and directional turns made is intended to be sufficiently accurate that the map displays the location of the vehicle relative to road intersections (i.e., approaching, passing trough, turning, etc.). With reference to FIGS. 16-17, 22, 27-28 and 30 of the Ando et al patent, the invention appears to put a premium on receiving GPS data that are accurate to within one meter.
A paging system with paging request receivers that respond or are controlled differently, depending upon receiver location, is disclosed by Vrijkorte in U.S. Pat. No. 4,943,803. Using "angle modulation" whose operation is not explained, the paging request receiver is said to pick out the strongest transmission control signal from among simultaneously transmitted control signals and to respond to this signal, if the signal received includes the receiver's predetermined address code. The receiver goes into a "sleeper" mode if no signals specifically addressed to that receiver are received within a selected time interval. The receiver is activated for receiving a control message by receipt of a receiver activation signal that may vary with the geographic zone presently occupied by the receiver.
Nishikawa et al disclose a land vehicle navigation system in U.S. Pat. No. 4,949,268. A combination of three or more GPS satellites is selected, from among all available GPS satellites, that provides the best vehicle location data. Here, a figure of merit used is minimum position dilution of precision ("PDOP") associated with the data received from a given group of GPS satellites. This approach seeks to account for the possibility that one or more buildings or other structures of relatively great height may preclude satellite visibility. The height of the structures in that area may be stored as part of a map in a GPS processor on board the land vehicle whose location is to be determined. The on-board system seeks combinations with the highest number of satellites visible, then works from these combinations to minimize the PDOP or a similar figure of merit for GPS position data.
Scribner et al disclose a vehicle tracking system that records, but does not transmit, the location of a vehicle whenever one or more predetermined events occurs in U.S. Pat. No. 5,014,206. The vehicle carries sensors that respond to occurrence of a predetermined event and carries a GPS or LORAN navigational system that receives vehicle location information, such as longitude and latitude. This vehicle location information is stored in a memory on board the vehicle only when one or more of the predetermined events occurs. The vehicle location information is assumed to be read out periodically when the vehicle returns to a home base.
In U.S. Pat. No. 5,043,736, Darnell et al disclose use of a combined cellular telephone and GPS portable receiver system that provides latitude and longitude coordinate information for the receiver. The GPS receiver has a modem and transmitter connected thereto that transmit its position to an accessible cellular phone, which in turn notifies a stationary base unit of the location of the GPS receiver. The base unit graphically displays the present location of the GPS receiver. This patent does not disclose determination of the location of the cellular telephone with reference to a prescribed cellzone boundary.
On-board navigation apparatus, to be carried on a vehicle to compare present and immediate-past locations as determined from GPS data, is disclosed by Odagawa et al in U.S. Pat. No. 5,087,919. A map containing coordinates for all roads in a region of the Earth's surface is stored electronically in a navigation system on board the vehicle, as part of a "map" of the region. The navigation system apparently assumes that the vehicle stays on one of the roads. Using the continuously arriving GPS data, the system searches for the nearest road segment that is consistent with these data to determine present location of the vehicle. Only latitude and longitude data are needed here so that fewer than four satellites may be used to determine the present location. Elevation data are provided as part of the road description stored in the navigation system. This system would not work well for a vehicle, such as an off-road land vehicle or a marine vessel, that has no fixed system of roads or paths to follow. This approach would not be particularly useful in determining the location of a stolen vehicle, unless the thief could be relied upon to stay on the conventionally describable roads.
Lojack Corp. of Dedham, Mass. has demonstrated a vehicle location system using triangulation of radio signals transmitted from the vehicle by a large number of receivers that are strategically positioned around a community. The vehicle transmitter is remotely activated by a signal broadcast from elsewhere.
Selby, in U.S. Pat. No. 4,876,738, and Comroe et al, in U.S. Pat. No. 5,054,110, disclose a registration procedure for a mobile cellular telephone user with one or more cellular phone service facilities that service zones in which the user operates. Where telephone contact with a cellphone user, who has not registered, is sought, the present location of this user must first be determined by area-wide paging. Registration of a cellphone user in a particular zone does not indicate that this user is presently in that zone.
Use of signals broadcast by a plurality of fixed site radio transmitters and received by a mobile cellphone user, to determine the user's present location by signal strength triangulation, is disclosed in U.S. Pat. No. 4,907,290, issued to Crompton. Location of the mobile receiver by this means does not indicate which, if any, cellular service provider can provide cellular communications service, and the typical location inaccuracy of this approach is probably many tens of meters.
Lee, in U.S. Pat. No. 4,932,049 and No. 5,067,147, discloses use of a main antenna in each cell zone plus several subsidiary antennas located on the cell zone boundaries or elsewhere to determine when a mobile cellular phone user has left that cell zone. Signal strengths from the mobile cellular phone are measured at each antenna in the cell zone and are compared with one or more reference sets of signal strengths to determine when handoff to an adjacent cell zone is appropriate.
U.S. Pat. No. 4,966,115, issued to Marui et al, discloses a mobile cellular phone system in which the strength of a signal received from a mobile cellular user are compared against first and second threshold values. If the signal strength is less than the first threshold value but greater than the second threshold value, a cell zone boundary is being approached. If the signal strength is less than the second threshold value, the mobile cellular user is determined to have crossed the cell zone boundary.
A method of predicting cell zone handoffs for a satellite-based cellular phone communications system is disclosed by Bertiger et al in U.S. Pat. No. 5,161,248. The geographic location of a mobile cellular phone user within a cell zone is determined, by means not discussed in detail. For a mobile user near a cell zone boundary, bit error rates for the satellite signals are computed and used to enhance the accuracy of the mobile user's calculated location.
In U.S. Pat. No. 5,175,867, issued to Wejke et al, a system for neighbor-assisted cell zone handoff is disclosed. A cellular service provider in a first cell zone measures the strength of signals received from a mobile cellular user that is moving in a second, adjacent cell zone. When this signal strength rises above a selected threshold, the first cell zone service provider requests handoff of the mobile user from the second cell zone to the first cell zone.
Bamburak discloses a location registration system for cellular mobile phones and other personal communicators in U.S. Pat. No. 5,197,092. The personal communicator includes means for call forwarding and updating thereof, stored at a central station through which calls made to the communicator are directed. This patent includes a useful discussion of cellular phone location registration approaches.
A method of controlling cellular mobile phone handoff by monitoring the Rayleigh multipath fading of the strength of a signal transmitted by a mobile cellphone to estimate the velocity of the cellphone is disclosed by Kanai in U.S. Pat. No. 5,239,667. This velocity estimate is used to estimate a difference between a reference signal strength level and the present signal strength level. When this difference reaches a selected threshold, cellphone handoff is implemented.
U.S. Pat. No. 5,241,685, issued to Bodin et al, discloses a load sharing technique for a mobile cellular phone system. Adjacent cellphone cell boundaries are defined in terms of relative signal strength. Thresholds for handoff, stated in terms of relative signal strengths, are dynamically adjusted in times of heavy communication traffic to lower the load on an overloaded cell and to increase the load on adjacent, less heavily loaded cells, by decreasing the area covered by the most overloaded cell.
Bissell et al, in U.S. Pat. No. 5,243,645, disclose a system for automatic forwarding of phone calls to a mobile recipient of such calls. When, or if, the recipient engages in a commercial transaction, such as use of a credit card or of an Automatic Teller Machine, that indicates the recipient's present location, this present location information is communicated to a central database. Phone calls are then automatically forwarded to the most recently communicated "present location" of the recipient. If the recipient has not engaged in such a commercial transaction, no present location information is available for call forwarding.
Koma et al disclose a cellular phone system with time display in which the mobile cellphone user continually receives a signal identifying the area code of the present cellzone of the user, in U.S. Pat. No. 5,258,964. If an Area Code boundary is crossed and this crossing involves a time zone change, this time change is entered manually initially and stored for possible future use. if this zone crossing occurs again. However, no details are disclosed for determining the present location of the cellphone or the Area Code zone in which the cellphone is located.
A mobile cellular phone registration system is disclosed in U.S. Pat. No. 5,289,527, issued to Tiedemann. A mobile user registers in a new cell whenever the user's distance from a selected location in the cell of present registration is greater than a threshold distance. This appears to require that all cells be of similar shape (e.g., regular hexagons) and size and does not necessarily provide an accurate indication of when the mobile user has crossed a cell boundary line. Relative range measurements are also used to determine the proper cell zone in U.S. Pat. No. 4,667,202, issued to Kammerlander et al.
In U.S. Pat. No. 5,293,643, Israelsson discloses a method for determination of handoff between two mobile cellular phone cell zones. When handoff from a first cell zone to another cell zone is allowed but not required, the correlation of a signal strength vector, whose vector components are the measured strengths of signals received from each of two or more adjacent cells at the mobile unit, with two or more reference signal strength vectors is calculated. These correlation values are compared to determine when, and to which cell zone, handoff is required. This approach requires careful determination of correlation threshold values for several representative locations within several cell zones, and adjustment of one or more cell zone boundaries may require remeasurement of many of these threshold values.
Sawyer et al, in U.S. Pat. No. 5,307,400, disclose a mobile cellular phone call routing routing approach that relies upon voluntary registration of the mobile user in a visited cell zone that is not the home cell zone. After voluntary registration in the visited cell is completed, the cellular service provider for the visited cell advises the home cell zone of this registration. The home cell zone cellular service provider then temporarily routes all phone calls for the mobile user to the visited cell zone cellular service provider. A somewhat similar approach is disclosed in U.S. Pat. No. 4,972,456, issued to Kaczmarek et al.
These patents disclose use of GPS or radiowave technology for determination of location or orientation of a person or vehicle continuously or at discrete times, without regard to which of several geographically defined zones the person or vehicle may have last entered. What is needed is a system that automatically tracks the movement of a vehicle or person, such as a cellphone subscriber, who moves from one cellzone to another, irrespective of whether a given cellphone subscriber is actively using cellular phone service, as a boundary is crossed between two adjacent cellzones. Preferably, this system should accurately determine which cellzone a given subscriber currently moves in and should be flexible enough to cover cellzones of arbitrary and changeable sizes, shapes and densities.